Sahih Muslim Book 4, Hadith Number 962.

Chapter : Moderation in the articles of prayers and their shortening and perfection.

‘Amr b. Huraith reported: I said the dawn prayer behind the Apostle of (may peace be upon him) and heard him reciting: ‘Nay. I call to witness the stars, running their courses and setting” (Al-Qur’an, lxxxi. 15-16) and Done of us bent his back till he completed prostration.